This invention relates to an aluminum electrolytic capacitor which utilizes an electrolyte comprising a mixed solvent system of N-methylpyrrolidinone and up to 5 wt% ethylene glycol plus a mixed solute system of dimethylammonium or diisopropylammonium boro-di-catecholate and diisopropylammonium pentaborate to provide a capacitor capable of operation at 150.degree. C. The solute system may be prepared as two separate solutes or prepared partially or completely in-situ from its components.
In the past, electrolytes that are based predominately on ethylene glycol have proved extremely useful at temperatures up to about 100.degree. C., generally 85.degree. C. For temperatures above 85.degree. C., the solvent choice has been dimethylformamide (DMF) usually, and capacitors containing DMF electrolytes have been valuable for operation up to 125.degree. C.
However, DMF is an agressive solvent, and containing DMF electrolytes in capacitors at operating temperatures above 125.degree. C. has been a distinct problem. A second problem has been the hydrolysis of DMF at 125.degree. C. and above.
Thermal stability of solutes at 125.degree. C. and higher has posed yet another problem. Not only must the solutes not decompose at these temperatures, they must also not react with other electrolyte components or with other capacitor materials. In particular, they must not participate in unfavorable reactions with the barrier layer dielectric oxide at the elevated temperatures.
The demand for capacitors capable of operating at 125.degree. to 150.degree. C. or higher has presented problems to capacitor manufacturers involving electrolyte stability, reactivity, and containment, as well as capacitor materials selection. The solution to these problems involves choices in electrolyte composition and capacitor materials selection.